As a way to preserve confidentiality of printed images, one type of an image forming apparatus that uses a thermal transfer ink ribbons melts the ink left on the ink ribbon or the ink ribbon itself after the image transferring process such that the image left on the ink ribbon cannot be recognized. Also, there exists image forming technology that allows a sheet with a printed image to be reused by printing with a decolorizable ink and erasing the printed image using heat. However, techniques for preserving confidentiality of images printed with decolorizable ink have not been developed.